June 2021 - 18

LIGHTING
Not All DLIs
are the Same
How can you achieve increased growth
with the same amount of light?
BY ANDRÉS MAYORGA-GÓMEZ
AND MARC VAN IERSEL
S
unlight gets scarce during the winter season. However, many plants
are grown in winter for sale during the busy spring season. Electric
lights, either HPS or LEDs, may be needed to supplement the light
that plants get naturally to ensure on-time production of high-quality
plants. Light requirements of horticultural crops are typically expressed
based on the daily light integral (DLI, total amount of light received in
a day).
The DLI provides a target for how much supplemental light needs to be
provided, but this supplemental light comes with significant electricity
costs, increasing production expenses. Research at the University of
Georgia found that not all DLIs are the same: greenhouse crops grow
faster with lower photosynthetic photon flux density (PPFD, the amount
of photosynthetic light reaching the plant) and longer photoperiods. This
can result in significant lower costs and higher revenues.
HOW THIS WORKS
We all know that plants need light for photosynthesis. But not all the
light that reaches the plant is used for photosynthesis. When plants
receive more light, they use it less efficiently. Since not all species have
the same light use efficiency, they do not all benefit from supplemental
light the same way.
18 JUNE 2021 GPNMAG.COM
But what is light use efficiency?
Imagine a kitchen sink. When you turn the
water on slowly, some water comes out of the faucet and
most of it is drained rapidly. With a low water flow, the sink is not going to
fill up. When you start increasing the water flow, the sink may no longer be
able to drain all the water and the sink may start filling up. Now imagine
that you start pouring buckets of water into the sink at once. The sink will
fill, and the water will spill over the sides. With too much water coming in,
the sink will exceed its capacity to drain the water.
Something similar happens with light and plants. Light is absorbed
by chlorophyll and other pigments in the leaf which creates electrical
currents, which in turn produce energy that is used to power
photosynthesis. At a low PPFD, much of the light can be used to create
electrical currents and power photosynthesis. Light is used efficiently,
just like the sink can drain a slow water flow efficiently. However, when
a plant receives more light, a smaller fraction of that light is used in
photosynthesis; most of the excess light energy is converted into heat.
Additionally, a small fraction of that light energy is converted into
fluorescence. Plants that are exposed to light give off a faint glow in

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June 2021

Table of Contents for the Digital Edition of June 2021

June 2021 - 1
June 2021 - 2
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